Respiratory syncytial virus (RSV) is a non-segmented, negative strand RNA virus of the Order designated Mononegavirales. Specifically, RSV is a member of the family Paramyxoviridae, genus Pneumovirus (1, 2). Respiratory tract disease caused by RSV imposes a significant burden on healthcare and all age groups are infected. The most significant disease however, occurs in young infants, aged adults, and patients with immunological abnormalities. It is estimated that lower respiratory tract (LRT) disease caused by RSV is responsible for 90% of bronchiolitis in infancy and 50% of all cases of pneumonia during the first two years of life. Thus, there is an urgent need for immunogenic compositions against RSV. Both subunit and live-attenuated immunogenic composition strategies have been followed to prevent LRT disease (3, 4). Unfortunately, neither tactic has thus far produced an acceptable product. The recent advent of “reverse genetics” technology, however, brings great promise for future immunogenic compositions (5). With “reverse genetics,” recombinant RSV strains may be genetically engineered with defined mutations to ensure an attenuated phenotype, or include genes encoding cytokines to modify adaptive immune responses. One caveat, however, is that replication of recombinant RSV in the airways may generate inflammatory responses that lead to wheezing in susceptible infants and toddlers. It is well documented that RSV bronchiolitis is a major risk factor for wheeze up to age 13 (6) and it is even suggested to set in motion immunological events that contribute to asthma (7).
The exact mechanisms whereby RSV infection brings about wheeze and asthma-like symptoms are unknown. It is likely that both innate and adaptive immune responses are involved. Several reports suggested that type 2 T cell responses were dominant in human infants with LRT disease caused by RSV. Peripheral blood eosinophilia, RSV-specific IgE and IgG4, and increased secretion of IL-4 from peripheral blood mononuclear cells (PBMC) stimulated with allergen or mitogen were associated with acute bronchiolitis caused by RSV (8, 9, 10). Because type 2 T cell responses and atopy are, key factors in asthma (11), unbalanced T cells responses against RSV antigens could contribute to harmful airway inflammation.
An antigen of primary interest in eliciting unbalanced T cell responses is the RSV attachment (G) protein. G protein is a heavily glycosylated 90-kDa type II transmembrane protein that is synthesized in both secreted and membrane-bound forms and has an important role in attachment of RSV to the host cell. The findings from several laboratories established that immunization with highly purified native or vaccinia virus-expressed recombinant G protein primed naïve BALB/c mice for pulmonary eosinophilia upon subsequent challenge with infectious RSV (12, 13). Eosinophilia was dependent on the presence of IL-5 and CD4+ T cells. In contrast, immunization with vaccinia virus-expressed RSV fusion (F) protein (12, 13), or appropriately adjuvanted natural F protein (14), did not prime for eosinophilia. Therefore, it may be possible to increase the safety profile of live attenuated immunogenic compositions or heterologous-expressed antigens of RSV through identification and deletion of G protein antigens that contribute to type 2 T cell responses. These antigens were identified in four inbred strains of naïve mice following immunization with native G protein (15). In a majority of strains, the responsible epitopes were located within the ectodomain encompassed by amino acids 149 to 200. Peptide-mapping studies further revealed that PBMC from most human donors readily recognized T cell epitopes present in amino acids 149 to 200 of G protein (16).